Heretofore, acrylonitrile composite fibers have been produced industrially as fibrous materials for clothes, carpets, etc., as having excellent crimp characteristics, high bulkiness, excellent dyeability, etc. As one example of producing them, there is known a method for producing side-by-side acrylonitrile composite fibers comprising composite components bonded together in a side by side relationship, in which a spinning dope comprising at least two acrylonitrile copolymers dissolved therein and differing in the content of sulfonate groups (water reversible components), as the components for forming the composite fiber, is let to a spinnerette for bicomponent spinning and spun therethrough (for example, Japanese Patent Publication No. 35288/82).
The water reversibility as referred to herein means such a property of reversible elongation due to swelling with water and shrinkage due to drying.
However, since the water reversible components are arranged in a laminar state throughout the fiber length in such a prior art, the crimps of the fiber are formed uniformly, being different from those of fibers of natural wool which are naturally varied in the axial direction of the fiber. Therefore, the hand of conventional acrylonitrile composite fibers is hard, being different from that of fibers of natural wool. The dyeing speed of acrylonitrile fibers having water reversible crimpability is accelerated with the enlargement of the water reversibility because of their sulfonate groups, with the result that they are often dyed unevenly to have dyeing specks in commercial dyeing in which, therefore, fibrous products made of the fibers cannot be dyed satisfactorily although conventional acrylonitrile fibers are generally dyed well.
We, the present inventors repeated our studies so as to overcome the above-mentioned drawbacks in the prior art and, as a result, have achieved the present invention.